Conventional flat fluorescent lamps comprise a flat glass container which is formed by a front glass panel 1, rear glass panel 2 and an unillustrated glass frame as shown in FIG. 6. The front glass panel 1 is formed on its inner surface with a phosphor coating 9, over which is provided an anode 7 having the function of a metal back formed by the vacuum evaporation of aluminum. A high voltage supply portion (not shown) is disposed at the end of the anode 7 with a carbon paste or the like provided therebetween.
Arranged inside the glass container are a plurality of linear cathodes 5 supported by posts 4, and a pair of mesh electrodes 6a, 6b serving as grid electrodes. The first mesh electrode 6a controls the amount of electrons to be supplied from the linear cathodes 5 to the phosphor coating 9 to which a high voltage is applied. A voltage lower than that applied to the anode 7 is applied to the second mesh electrode 6b to permit the first mesh electrode 6a to block electrons. The energy of the electrons from the linear cathodes 5 passes through the anode 7, exciting the phosphor coating 9 to cause the coating to luminesce with a high luminance.
Indicated at A and B in the drawing are opposite ends of the linear cathodes 5, and at C is the midportion of each cathode. If a d.c. voltage is applied to the linear cathodes 5, a potential gradient occurs between the cathode ends to create irregularities in luminance. Accordingly, sine-wave voltages reverse to each other in phase are applied to the respective ends of each linear cathode 5 as seen in FIG. 7. In FIG. 7, the solid line represents the waveform of voltage applied to one end A of the cathode 5, and the broken line represents the waveform of voltage of reverse phase to the above applied to the other end B of the cathode. At this time, the midportion C is grounded.
FIGS. 8 to 13 show the amount of electrons passing through the mesh electrodes in the case where a d.c. voltage is applied to the mesh electrode 6a and is varied to control the intensity of light of the lamp as in the prior art. In these drawings, plotted as abscissa is time T vs. the amount I of passage of electrons as ordinate. The amounts of passage of the electrons generated from the portions A, B and C during high-luminance luminescence are represented in FIGS. 8, 9 and 10, respectively, by hatching. The amounts of passage of the electrons produced from the portions A, B and C during low-luminance luminescence are represented in FIGS. 11, 12 and 13, respectively, by hatching. During the high-luminance luminescence shown in FIGS. 8, 9 and 10, the portions A, B and C are all generally uniform in the amount of electrons passing through the mesh electrodes.
However, the low-luminance luminescence shown in FIGS. 11, 12 and 13 involves the problem that the end portions A and B produce higher brightness than the midportion C to result in uneven luminance since when the voltage applied to the end of the linear cathode 5 is -3 V (see FIG. 7), the greatest potential difference results relative to the mesh electrode 6a. Especially, FIGS. 11 to 13 show little or no luminescence at the midportion in contrast with luminescence occurring at the end portions.
On the other hand, Unexamined Japanese Patent Publication SHO 56-19861 discloses a flat fluorescent lamp of the transmission type as shown in FIG. 27.
As illustrated, a flat glass container is formed by a front glass panel 1, a rear glass panel 2 and a glass frame 3.
Arranged inside the glass container are a plurality of linear cathodes 5 supported by posts 4, a mesh electrode 6 serving as a grid electrode, a transparent electrode 7 made of tin oxide or indium oxide and formed on the inner surface of the front glass panel 1, a high voltage supply portion 8 at the end of the anode, and a phosphor coating 9 formed on the anode. The linear cathodes 5 emit electrons, which excite the phosphor coating 9 and cause the coating to luminesce with a high luminance.
The fluorescent lamp has the drawback that the light from the phosphor coating 9 is emitted also through the rear glass panel 2.
FIG. 28 shows another flat fluorescent lamp disclosed in Unexamined Japanese Patent Publication SHO 63-10458 and serving as a backlight for liquid crystal panels.
As illustrated, a front glass panel 1, a rear glass panel 2 and glass frame 3 form a flat glass container which has arranged therein a plurality of linear cathodes 5 supported by posts 4, a mesh electrode 6 serving as a grid electrode, a phosphor coating 9 formed on the inner surface of the front glass panel, and an anode 7 formed as a metal back with aluminum by vacuum evaporation on the phosphor coating 9. A high voltage supply portion 8 is provided at the end of the anode 7 with a carbon paste or the like disposed therebetween. The energy of the electrons from the linear cathodes 5 passes through the anode 7 and excites the phosphor coating 9 to cause the coating to luminesce with a high luminance.
With this fluorescent lamp, the light emitted from the phosphor coating toward the rear panel 2 is reflected at the metal back and directed toward the front glass panel 1.
However, the fluorescent lamp has the drawback that the energy possessed by electrons is consumed when passing through the metal back to result in a reduced efficiency.
Further with either of the flat fluorescent lamps shown in FIGS. 27 and 28, discharge is likely to occur between the post 4 and the high voltage supply portion 8 of the anode 7 to entail the problem that a voltage of sufficiently high value is not supplied to the anode, failing to ensure satisfactory luminescence performance.
Further because electrons are released straight only from the portions of the linear cathodes 5 facing the front panel toward the mesh electrode 6 almost without spreading, the amount of electrons at the intermediate portion between the two adjacent linear cathodes 5, 5 is small. This leads to the problem that a dark area is produced.
Still another problem is encountered in that the energy of the electron beam partly becomes a heat loss to overheat the phosphor coating 9 to deteriorate the coating.